Method of coating leather



United States Patent 3,147,138 METHOD OF COATING LEATHER Erwin Muller,Wolfgang Speicher, and Peter Coppe, Leverkusen, and Rudolf Merteu,Cologne-Flittard, Germany, assignors to Farbenfabriken BayerAktiengesellschaft, Leverkusen, Germany, a corporation of Germany NoDrawing. Filed Jan. 16, 1962, Ser. No. 166,672 Claims priority,application Germany Jan. 28, 1961 7 Claims. (Cl. 117-142) This inventionrelates to coated leathers and to proc esses for producing the same.More particularly, it relates to an improved primary or bottomingcoating for leathers.

When leather is used as a material for various everyday articles, it isgenerally provided prior to processing with coatings which are usuallydyed or pigmented. These dressings are intended to impart a pleasingappearance to the leather and to protect it as far as possible frommoisture and mechanical stresses. The natural character of the leather,for example, grain strength, softness and flexibility, should not beimpaired by such dressings. In order to obtain permanently flexible andnevertheless mechanically resistant coatings on leather, it is necessaryin most cases to build up the dressing in layers, starting with abottoming, which is frequently arranged to be soft, and terminating witha finishing layer which is usually hard.

The bottoming layer, which in many cases is decisive as regards thequality of the entire dressing, has various objects according to thetype of leather being treated, of which a few are set out by way ofexamples below.

It should first of all serve as a base for the succeeding dressinglayers which are usually harder and prevent the latter from penetratingtoo deeply into the leather and thus counteracting a hardening of theleather material. Furthermore, it must provide a bond between theleather material and the upper dressing layers. Consequently, it isessential for the adhesion of the entire dressing. By a correctlyadjusted degree of softness and a depth of penetration adapted to theleather material, the bottoming can favorably influence the feel,softness, and associated of the grain on the leather. The bottoming isfrequently of decisive importance as regards the mechanical propertiesof the complete dressing, such as resistance to cracking, resistance tocold and fastness to friction glazing.

As the bottoming or primary layer is of great importance with respect tothe quality of the finished leather article, numerous methods have beenproposed with respect to the bottoming systems for leathers. It has beenheretofore known to introduce bottomings containing oil, wax or fatusually in an aqueous emulsion or solution in order to have aparticularly favorable influence on the softness and grain associationof the leather. Nevertheless,lthe bonding strength of the dressing filmcan be impaired by such bottoming layers. Furthermore, polymers, forexample, esters of acrylic acid or copolymers with butadiene, usually inaqueous emulsion, have frequently been used for the bottoming ofleather. Watersoluble or organically soluble natural or syntheticmaterials, such as casein, albumin, vegetable mucins, shellac, alkydresins, nitrocellulose and the like have often been recommended asbottoms for leathers. These substances, however, frequently do notconform in their softness to the standards which must be set for leatherbottoming layers. Combinations of the said types of bottoming layers,often in combination with pigments or fillers, can on the contrary leadto bottoming layers which are successfully employed in practice andwhich do not disadvantageously influence the natural properties of theleather, particularly the grain association.

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It has also been known to improve the grain association ofhollow-grained leather to use as bottoming layers, solutions ofunmodified alkyd resins having free hydroxyl groups in organic solvents,which also contain crosslinking agents, such as isocyanate, see forexample, German Patent 889,349. The said urethane based prepolymerswhich are used in solution in organic solvents, have the property ofstrengthening the bond between the grain layer of the leather and therecticular layer. This is shown in a better grain association, that isto say, when the leather is bent with the grain layer on the inside, thegrain layer is warped into very fine corrugations, even if a coarsecreasing, caused by a loose bonding prior to the treatment of theleather With the polyurethanes, using the same test. The consolidationof the grain by the said polyurethanes also leads to an improvedresistance to scratching and fastness to friction glazing of thedressing. This is shown when a sharp-edged article is moved underpressure over the dressed leather. The untreated areas become warped incorrugated form, while the leather treated with the polyurethanesremains smooth.

One disadvantage of the use of the previously described polyurethanes isthat the organic solvents necessary to dilute them are combustible, cancause physiological injury, can initiate undesired migrations of fattysubstances in the leather and in addition substantially increase thecost for dressing or currying. Where the polyurethanes still containfree isocyanate groups, they are in addition restricted as regards theirstorage capacity and above all are very sensitive to moisture.

It is, therefore, an object of this invention to provide an improvedprocess for bottoming leather. It is another object of this invention toprovide an improved bottoming system for leathers which does not impairthe bonding strength of the subsequently applied layers to the leathers.It is still another object of this invention to provide leatherbottomings which consolidate the grain of the leather to a high degree.It is a further object of this invention to provide a bottoming layer toleather which can be applied from an aqueous solution. It is a stillfurther object of this invention to provide leather having an improvedresistance to friction glazing.

The foregoing objects and others which will become apparent from thefollowing description are accomplished in accordance with thisinvention, generally speaking, by applying to the leather an aqueoussolution of a Water soluble reaction product of a polyhydric alcohol andan aldehyde or an organic polyisocyanate or an aldehyde and apolyisocyanate. The water soluble condensation products according to theinvention are suitable for bottoming layers and also for barrier layerson leathers which are subsequently to be dressed or curried withreaction lacquers comprising polyfunctional isocyanates andpolyfunctional soft resins containing active hydrogen atoms.

The bottoming layers in accordance with this invention can also beprepared by reacting a polyhydric alcohol with an aldehyde or by firstreacting the polyhydric alcohol with the aldehyde and then reacting theproduct with an organic polyisocyanate. In either event thepolyisocyanate is used in an amount insuflicient to react with all ofthe hydroxyl groups present to thereby insure that hydroXyl groups willterminate the polymer forming the bottoming layer. The resulting polymeris then dissolved in water or an aqueous solution. The use of thebottoming or priming layers in accordance with this invention in anaqueous solution greatly simplifies the practice of coating leathers andconcurrently reduces the cost of the process over the alkyd typecoatings which require organic solvents, while greatly increasing thebonding strength of subsequent layers to the leather.

Any suitable divalent or polyvalent hydroxyl compound free of estergroups may be used in reaction with an aldehyde, an isocyanate, or analdehyde and an isocyanate in accordance with this invention such as,for example, ethylene glycol, propylene glycol, butanediols,hexanediols, glycerine, trimethylol alkanes, pentaerythritols,tetramethylolcyclohexanol, sorbitol, glucose, saccharose, invert sugar,acidly degraded starches, thiodiglycol and the like; adducts of alkyleneoxides such as, for example, ethylene oxide, propylene oxide, butyleneoxide, amylene oxide and the like with any of the aforesaidpolyalcohols, polyfunctional amines or amino alcohols such as, forexample, ethylene diamine, propylene diamine, hexamethylene diamine,3,3'-dichlorobenzidine, 2,5-dichloro-p-phenylene diamine, hydrazine,phenylene diamine, tolylene diamine, 4,4 diamino diphenyl methane,aminoethyl alcohol, aminopropyl alcohol, aminobutyl alcohol and thelike; polyalkylene ethers such as, for example, diethylene glycol,triethylene glycol, octa-ethylene glycol, polyalkylene glycols such aspolyethylene glycol, polypropylene glycol with molecular weights inparticular between 500 and 2000, oxalkylated glycerine, trimethylolpropane, sorbitol, carbohydrates, hydroquinone, ethylene diamine, mixedethers of thiodiglycol and ethylene glycols and the like. Monofunctionalalcohols can also be concurrently used in minor proportions.

Further, an additional modification consists in the concurrent use ofphenols, more especially phenol, cresols, ethylphenols or tertiarybutylphenols, during further or initial lengthening with aldehydes. Thecondensation products which are claimed are produced by lengthening theaforesaid polyhydroxyl compounds with aldehydes and/or polyisocyanatesand it is also possible for several of the aforesaid lengthening agentsto be used simultaneously or in a timed sequence for the lengtheningoperation.

Any suitable aldehyde may be used such as, for example, formaldehyde inits various forms, acetaldehyde, butyraldehyde, acrolein,crotonaldehyde, furfurol, glyoxal and the like. Other compounds whichsplit off formaldehyde may also be used. For successfully conducting thecondensation acid catalysts such as sulphuric acid, sulphonic acids,phosphoric acid, fluoboric acid or perchloric acid should be added incatalytic quantities and preferably in the amount of 0.01 to by weight.

Any suitable organic polyisocyanate can be used for the production ofthe water soluble polycondensation products such as, for example,hexamethylene diisocyanate, decamethylene diisocyanate,1,7-diisocyanato-4-oxa-heptane, o-, mand p-phenylene diisocyanate,toluylene diisocyanates and their isomeric mixtures, diphenyl methanediisocyanate, naphthalene diisocyanates, cyclohexylene diisocyanate,furfurylidene diisocyanate, 4,4,4"-triisocyanato-triphenyl phosphate andother compounds as well as reaction products containing NCO groups andobtained from the above polyisocyanates with polyalcohols or partialpolymers of the above polyisocyanates and the like. When using thecompounds containing isocyanate groups, care must be taken that thereaction of the polyisocyanate is carried out as far as possible in theabsence of water.

The proportions of the polyhydroxyl compounds and lengthening agents,that is the aldehyde and/or polyisocyanate, should be such that thereaction products obtained still contain free hydroxyl groups and havehydroxyl numbers preferably between 1 and 800. It is essential that thepolycondensates obtained should be soluble in water, and this can beachieved either by a corresponding accumulation of hydroxyl groups or bythe concurrent use of hydrophilic components, such as di-, tri-, orpolyethylene glycols in the condensation. The preparation of some of thepreviously described condensation products is described by way ofexample in US. Patent 2,961,428 and in US. application Serial No.71,175.

The bottoming or priming agents according ot the invention are usuallyapplied to the grain side of the undressed, buffed or unbufr'ed leatherin an aqueous solution either once or several times by the conventionalmethods, by hand, by spraying, brushing, or by using a machine. Thequantity and the concentration of the coating applied depends on thedegree of hollow graininess and the absorption capacity of the leatherto be treated. After drying the bottoming, the leather can be ground,polished or ironed. The dressing of currying is thereafter effected withnormal commercial surface colors and binding agents or with reactionlacquers consisting of polyfunctional soft resins containing hydroxylgroups in the normal manner.

Wetting agents, solvents and emulsifiers can be added to the bottomingor priming layers to be used according to the invention for regulatingtheir penetration power. Dyestuffs or pigment preparations can beconcurrently used for coloring the priming or bottoming agents. It isalso possible simultaneously to use other auxiliaries for the dressingof leather, such as polymer emulsions, casein solutions and others,which intensify the barrier layer effect of the priming solutionsaccording to the invention.

While any of the compositions set forth above are suitable in accordancewith this invention as bottoming layers applied to leather in an aqueoussolution, it is preferred that the bottoming composition be prepared byreacting a triethylene glycol with formaldehyde and then reacting thiscomposition with a quantity of toluylene diisocyanate insufiicient toreact with all of the hydroxyl groups present.

The invention is further illustrated, but not limited by the followingexamples, in which parts and percentages are by weight unless otherwisespecified.

Example 1 A polyhydroxyl compound used in the bottoming or priming isprepared as follows:

About 1.5 kg. of triethylene glycol, about 340 g. of paraformaldehydeand about 3 g. of concentrated H are heated While stirring over carbondioxide to 80-90 C. in a flask provided with a reflux condenser, until aclear melt is formed, this taking about 1 hour. The water split off isthen extracted, initially under light vacuum and finally at a pressureof 12 mm./Hg and condensation is carried out at 80 to C. until no morewater distills over. The quantity of distillate is 214 g. Thecondensation product which has formed and which is a yellowishwater-soluble oil, has an hydroxyl number of 88. The 1.6 kg. ofpolyacetyl which are obtained are adjusted with alkali agents to pH 7.About 200 g. of toluylene diisocyanate are then added with stirring.After careful homogenization, about 1.6 cc. of tri-nbutylamine are addeddropwise. The mixture is heated for 4 hours to 80 C. After cooling, theproduct obtained is dissolved in water.

A 10% aqueous solution of this polyhydroxyl compound is applied by meansof a plush wheel or brush or by spraying to a chrome-tanned cowhidewhich is retanned with vegetable tanning agents and the grain side ofwhich is roughened with finegrain emery paper. After drying the bottompriming coat, the leather is pressed by hand or by the use of ahydraulic press. Thereafter, a conventional dressing or currying,consisting for example of casein-pigment dispersion and polymeremulsions, is carried out by the methods usual in practice. Theresultant dressed or curried leather is distinguished from a leatherdressed in the same manner but not with the bottoming agents accordingto the invention by an improved grain effect and an improved resistanceto friction glazing.

Example 2 A polyhydroxyl compound is prepared from about 10,000 parts ofcane sugar, about 23,000 pars of 40% formalin (81 g.=1 mol CH O), about19,100 parts of diethylene glycol, about 13,950 parts of trioxethylatedtrimethylol propane and about 300 parts of 30% fluoboric acid.

The pH value of the experimental batch is 2.7 in the form of its 50%aqueous solution. This mixture is heated for 4 hours to 90 C. and thewater which is added and which forms is then removed as far as possiblequantitatively at 90 C./12-18 mm. Hg. Dry ammonia is then introduced foranother 4 hours at 40 C. and the substances left for 1 hour at 40 C. ata pressure of 12 mm. Hg. C.P. ==4700; 10.5% OH; 0.4 acid number; 0.2% CHO; 0.58% H O, pH-value of 50% aqueous solution=8.6.

An aqueous solution for the bottoming composition is prepared from about100 parts of a normal commercial aqueous leather surface color with acasein base, about 250 parts of the above described polyhydroxylcompound, about 250 parts of water and about 200 parts of a normalcommercial aqueous polymer emulsion.

A buffed chrome calf leather is treated by hand or spraying with thepriming or bottoming mixture above described. After drying, the leatheris pressed by hand or by using a hydraulic press. This bottoming layerhas applied thereto, a reaction lacquer consisting of solutions ofpolyfunctional soft resins containing hydroxyl groups and the quantityof polyfunctional isocyanates required for the cross-linking, theapplication being made by spraying. After drying the reaction lacquer, alacquered leather is obtained, which is distinguished by a very finegrain formation, softness, good bonding strength of the lacquer andexcellent fastness properties.

Example 3 A 50% aqueous solution of a polyhydroxyl compound is preparedas follows:

Abous 500 g. of a polyethylene oxide having a molecular weight of about1,000 and an hydroxyl number of about 1 12 are neutralized with gaseoushydrochloric acid to remove the alkali still present from the productionof the product and, after dehydration at 130 C./ 12 mm. Hg, about 80 g.of toluylene diisocyanate are incorporated by stirring at about 90 C.The temperature is raised to from about 130 to about 140 C. After about2 hours, the viscous melt is poured onto metal sheets and heated forabout 12 hours at about 100 C. The thermoplastic material thus obtainedand having a rubber-like consistency is dissolved in about 586 g. ofwater.

Example 4 To an unbufied dyed chromium calf leather there is applied bymeans of a plush a aqueous emulsion of a copolymerisate frombutadiene-acrylonitrile-styrene which may be dyed with water-solubledyestuffs or caseinpigment dispersions. After drying of this coating,the base mixture described below is applied again by means of plush. Theleather is then dried by airing over night and subsequently pressed byhand or by means of a hydraulic press. To these bottom layers a reactionlacquer is applied by spraying which is prepared from solutions of softresins containing polyfunctional hydroxyl groups and of the quantity ofpolyfunctional isocyanates required for cross-linking. The enamelledleather obtained after drying of the reaction lacquer is excellentlysuitable for upper leather and bag leather on account of its goodfastness properties, fine grain and supple feel.

The base mixture mentioned above is composed as follows:

'15 parts of a 20% aqueous solution of the polyhydroxyl compounddescribed in Example 1,

parts of a 20% commercial aqueous emulsion of a polyamide from adipicacid-hexamethylene-diamine and/ or caprolactam,

200 parts of water.

It is, of course, to be understood that the invention is not to belimited by the working examples, but that any of the hydroxyl compoundsprepared using the compounds set forth above such as any of the hydroxylterminated compounds, aldehydes and isocyanates for those specificallyused in the working examples.

Although the invention has been described in considerable detail for thepurpose of illustration, it is to be understood that variations can bemade by those skilled in the art without departing from the spirit ofthe invention and scope of the claims.

What is claimed is:

1. A process for bottoming leather which comprises applying to theleather in an aqueous solution an bydroxyl terminated reaction productof a polyhydric alcohol and an organic polyisocyanate.

2. A process for bottoming leather which comprises applying to theleather in an aqueous solution an hydroxyl terminated reaction productprepared by reacting a polyhydric alcohol with an aldehyde in a firststep and reacting the reaction product of the first step with an organicpolyisocyanate.

3. A method for bottoming leather which comprises applying to theleather an aqueous solution of the reaction product of triethyleneglycol, formaldehyde and toluylene diisocyanate.

4. A process for bottoming leather which comprises applying to theleather in an aqueous solution an hydroxyl terminated reaction productof a polyhydric alcohol and an organic polyisocyanate, said hydroxylterminated reaction product being substantially free of ester groups.

5. A process for bottoming leather which comprises applying to theleather in an aqueous solution an hydroxyl terminated reaction productprepared by reacting a polyhydric alcohol with an aldehyde in a firststep and reacting the reaction product of said first step with anorganic polyisocyanate, said hydroxyl terminated reaction product beingsubstantially free of ester groups.

6. The process of claim 2 wherein the aldehyde is a lower aliphaticaldehyde.

7. The process of claim 2 wherein the aldehyde is formaldehyde.

References Cited in the file of this patent UNITED STATES PATENTS2,786,081 Kress Mar. 19, 1957 2,946,767 Gassmann July 26, 1960 2,956,963Baird Oct. 18, 1960 2,961,428 Muller et al. Nov. 22, 1960 3,066,997Neher et al Dec. 4, 1962

1. A PROCESS FOR BOTTOMING LEATER WHICH COMPRISES APPLYING TO THELEATHER IN AN AQUEOUS SOLUTION AN HYDROXYL TERMINATED REACTION PRODUCTOF A POLYHYDRIC ALCOHOL AND AN ORGANIC POLYISOCYANATE.